1. Field of the Invention
The present invention relates to a compression ignition engine.
2. Description of the Related Art
Known in the art is a compression ignition type engine having an NOx absorbent arranged in an engine exhaust passage and absorbing NOx when an air-fuel ratio of an inflowing exhaust gas is lean and releasing the absorbed NOx when the air-fuel ratio of the inflowing exhaust gas becomes the stoichiometric air-fuel ratio or rich, burning the fuel normally at a lean air-fuel ratio, having the NOx produced at that time absorbed in the NOx absorbent, and making the air-fuel ratio temporarily rich when the amount of NOx absorbed in the NOx absorbent exceeds a predetermined allowable amount and thereby having the NOx released from the NOx absorbent and reduced.
If the air-fuel ratio is made rich in a compression ignition type engine, however, a large amount of soot is produced. Therefore, it is considerably difficult to make the NOx be released from the NOx absorbent by making the air-fuel ratio rich.
On the other hand, if the recirculation rate of exhaust gas recirculation gas (hereinafter referred to as xe2x80x9cEGR gasxe2x80x9d) (amount of EGR gas/(amount of intake air+amount of EGR gas)), that is, the EGR rate, of a compression ignition type engine is made over 55 percent, the combustion temperature will fall and so-called low temperature combustion will occur. At that time, it is known that even if the air-fuel ratio is made rich, almost no soot will be produced. Therefore, if the air-fuel ratio is made rich at the time of low temperature combustion in this way, it is possible to release the NOx from the NOx absorbent without generating soot.
This low temperature combustion, however, is only possible when the engine load is light. When the engine load rises, low temperature combustion is not possible. Therefore, when the engine load is high, it is not possible to make the air-fuel ratio rich without generating soot.
Accordingly, known in the art is a compression ignition type engine provided with an electric motor for generating a drive power separate from the drive power of the engine, reducing the engine load for low temperature combustion when the air-fuel ratio should be made rich when the engine load is high, and operating the electric motor to compensate for the drop in the engine output due to the drop in engine load (see Japanese Unexamined Patent Publication (Kokai) No. 11-257054).
An electric motor, however, no longer operates well if the amount of electrical energy stored in the battery for driving the electric motor falls. Therefore, when the amount of electrical energy stored in the battery falls, it is necessary to stop the operation of the electric motor. In the above compression ignition type engine, however, no consideration at all is given to the amount of electrical energy stored in the battery, so sometimes the electric motor will not operate well when operating the electric motor. In such a case, the problem will arise of the drive power for the vehicle sharply falling.
An object of the present invention is to provide a compression ignition type engine capable of properly releasing NOx and SOx from the NOx absorbent by using the drive power of an electric motor.
According to the present invention, there is provided a compression ignition type engine in which an amount of production of soot gradually increases and then peaks when an amount of inert gas in a combustion chamber increases and in which a further increase of the amount of inert gas in the combustion chamber results in a temperature of fuel and surrounding gas in the combustion chamber becoming lower than a temperature of production of soot and therefore almost no production of soot any longer, the engine comprising: an exhaust passage; an NOx absorbent arranged in the exhaust passage, the NOx absorbent absorbing NOx when an air-fuel ratio of inflowing exhaust gas is lean, and releasing the absorbed NOx when the air-fuel ratio of the inflowing exhaust gas is rich or the stoichiometric air-fuel ratio; switching means for selectively switching between a first combustion where the amount of the inert gas in the combustion chamber is larger than the amount of inert gas where the amount of production of soot peaks and a second combustion where the amount of inert gas in the combustion chamber is smaller than the amount of inert gas where the amount of production of soot peaks; an electric motor for generating a vehicle drive power separate from a drive power of the engine; a battery for driving the electric motor; decision means for deciding if an amount of electrical energy stored in the battery is more than a predetermined amount; and control means for performing the first combustion, making the air-fuel ratio in the combustion chamber rich or the stoichiometric air-fuel ratio, and generating drive power at the electric motor even when the second combustion should be performed when NOx or SOx should be released from the NOx absorbent and the amount of electrical energy stored in the battery is more than the predetermined amount.